The Army wanted the device for rescue operations, like lifting wounded soldiers or hauling someone out of the water. Ball's idea was to build a battery-powered winch that someone could wear around their waist.

In 2005, he formed a company called Atlas Devices to work on the project, and eventually it paid off. "Twelve years later, with a lot of blood, sweat and tears along the way, Atlas Devices gets to build those powered ascenders," Ball says.

Atlas Devices sells the ascenders to the Army, and the Army uses them in rescue operations, as expected. So do fire departments and other first responders.

But Ball also noticed that utility companies started buying them, and when he contacted them to ask why, they explained they were using the ascender to install live power lines called conductors.

"They actually pick up the conductors and raise them up into the air and attach them to the tower," he says. It was an application he had never considered.

That experience led to a revelation.

"Sometimes you come up with a concept and put it out into the world with an initial vision for what it's good for, and how people are going to use it," he says. "Then they come up with new ways to use it. And that's one of the most exciting things about putting something new into the world, is you actually don't know what it might get used for."

Of course, there's a long history of inventions finding unexpected uses. For example, in 1953 the Rocket Chemical Company was trying to develop solvent that would displace water from the surface of metals. Their 40th attempt was successful, and the water displacement solvent was initially used to protect the outer skin of missiles, but WD-40 also became popular around the house as a lubricant.

Or the case of Pfizer chemists who were looking for a drug to treating high blood pressure and chest pain. The drug flopped for those uses, but Viagra became quite popular when used for other applications.

Ball says the same thing has happened with a seemingly straightforward invention: the ladder.

After Ball built the ascender, the military approached Atlas Devices about designing a ladder that was strong, but lightweight and segmented so it was easy to carry. "Ladders can still benefit from a lot of innovation," Ball says.

Ball and his colleagues designed and built such a ladder. The military liked it, but used it in unexpected ways. In addition to using it to climb, in at least one instance a group of soldiers took the device apart and turned it into a kind of stretcher.

Ball now considers this kind of thing a part of the invention process: designing new devices, but then working closely with the people who use them to figure out what they're really useful for. In the ladder example, he's now added straps to the ladder to make it easier to carry when used as a stretcher.

Copyright 2018 NPR. To see more, visit http://www.npr.org/.

RACHEL MARTIN, HOST:

NPR's Joe Palca thinks a lot about thinking - specifically how people come up with the big ideas that change our lives. And as you'll hear in this next story, sometimes the inventor himself doesn't even recognize all the possible uses of the thing they have created.

JOE PALCA, BYLINE: About 15 years ago, the U.S. Army came to the Massachusetts Institute of Technology with a request.

NATE BALL: Can somebody build a powered device that can pull somebody up a rope like Batman?

PALCA: That's Nate Ball. At the time, he was an undergrad at MIT.

BALL: We looked at each other and said, that sounds awesome. We'd love to build that.

PALCA: The Army wanted the device for rescue operations. Ball's idea was to build a kind of battery-powered winch that someone could wear around their waist. In 2005, he formed a company called Atlas Devices to work on the project.

BALL: Twelve years later, with a lot of blood, sweat and tears along the way, Atlas Devices gets to build those powered ascenders.

PALCA: Ball sold the ascenders to the Army. And the Army used them in rescue operations. So did fire departments and other first responders. But one day, he heard that utility companies were buying them, and Ball wondered why. So he contacted the companies, and they explained they were using the ascenders to repair power lines.

BALL: They're able to use the ascender to haul conductor, which means actually, like, pick up the power lines and, you know, raise them up in the air to attach them to the tower.

PALCA: Ball says he would never have thought of that application for the ascender. He says that's when he realized something important about inventing. Sometimes you come up with a concept and put it out into the world with an initial vision for what it's good for and how people are going to use it.

BALL: And then they come up with new ways to use it. And that's one of the most exciting things about putting something new into the world, is you actually don't know all the things that it might get used for.

PALCA: Ball says this even happens with a seemingly straightforward invention. After he built the ascender, the military came back looking for a better ladder.

BALL: Ladders can still benefit from a lot of innovation.

PALCA: The military wanted something strong but lightweight and segmented, so it was easy to carry. Ball and his colleagues designed and built such a ladder. The military liked it, and then...

BALL: We found out that in a - you know, in a difficult situation, they had actually taken apart the ladder. And they had extracted a person who had been injured.

PALCA: Turning the ladder into a kind of stretcher - again, not what Ball had in mind at all. But for him, that's now part of the invention process - letting people who use his new tools and devices figure out what they're really useful for. Joe Palca, NPR News.